Parasitic filter



May 1942- G. L. USSELMAN 2,284,131

PARASITIC FILTER 1 Filed March so, 1940 OUTPUT INVENTOR G-L. USSELMAN/V 'ATTORNEY Patented May 26, 1942 George L. Usselman,

notation of Delaware Port .Iefi'erson, N. Y;, assignor to Radio Corporation 01' America, a cor- Application March 30, 1940, Serial No. 326,932 I 4 Claims. (c1.'17s 171) This invention relates to an improved type of anti-parasitic filter which damps out the parasitic oscillations with least efiect on the normal operation of an oscillator or amplifier stage of the radio transmitter.

An object of this invention is to provide a simplified and improved filter system for damping out parasitic oscillations of an amplifier or oscillator stage.

The usual practice known in the prior art has been to connect a filter consisting of parallel connected inductances' point of a plate tank and resistances to the midcoil of a vacuum tube stage to prevent a type of parasitic oscillation. In some cases, the normal amount of oscillating current or the fundamental or harmonic frequency is great enough to cause considerable loss of power in the resistor. Quite often, it occurs that this parasitic circuit tends quency oscillation.

to generate much lower fre- Consequently, by my invention, I propose to improve the type of filter known in the prior art by connecting in parallel with the inductance and resistance a by-pass' condenser.

This type of filter would have a low impedance to the normal operating oscillating currents, but it would have a high impedance to the lower freting currents. This would combine low filter loss for normal operation and an eflective filter or choke for the parasitic frequencies. 1 This invention will best beunder'stood by referring to the accompanying drawing, in which the filter portion is shown as in parallel. The filter F is with the drawing plifier stage. An equivalent I may be any connection on apparatus or circuit which zero radio frequency potential ground. Inductance 2 has connected in parallel F and consists 'of a choke inductance L, a resistance R, erably non-inductive, and a fixed or These three elements are which is prefvariable conconnected in series shown by the push-pull amto the midpoint tap any high frequency is of substantially with respect to therewith a variable tuning condenser 3. The push-pull amplifier stage includes an exciter circuit 4 connected to an a midpoint tap being co ected to the grids tron disc arge devices 0 and II and I2 are connected to Grid bias is obtained from a I4 and a by-pass condenser tween the bias and cathode input inductance 5 having 5, the outer ends of inductance 5 I and 8 of two elec- III. The cathodes ground at point l3.

direct current source I5 is connected becircuits. a tuning of the plate condenser I6 is also connected to the outside ends 'of inductance coil 5. Grids l and 8 are neutralized by variable condenser I! and I8. The anodes l9 and 20 are connected to the outside ends of output inductance 2. The output of the amplifier is coupled to inductance 2 by means of a coupling coil 2|. The midpoint of inductance 2 is connected to one side of filter F, the other side of the filter being connected to the positive side of the B" supply and a by-pass'condenser 22 being connected in series from-one side of the filter to ground I 3. Push-pull amplifier stages, similar to that shown above, are often subject to what is sometimescalled "push-pull parasitic oscillations} in which the center tap connection I circuit is part of the parasitic circuit. One method of preventing this parasitic occurrence is to insert an impedance in the circuit at this point. It theimpedance is too large, it will interfere with the normal operation of the amplifier; if it is too small, it will not hinder the parasitic oscillations. The filter that will stop the parasitic oscillation is sometimes a large enough impedance to interfere with the efiiciency of the amplifier which depends on a ground path for the second harmonic current.

In the operation of the filter of this invention, the choke L carriessubstantially all the direct plate or anode current and chokes out substanfilter of this invention, for a frequency of 10,600,000 cycles per sec., for an amplifier, the inductance L should have a value of 150 micronenries, resistance R a value of 110 ohms, and the capacity of condenser C a value of 370 micromlcro-farads. Y r

Although this invention is shown as being applied only to a push-pull amplifier, it is to be distinctly understood that it will also equally apply to an oscillation circuit. Therefore, it should not be limited to the specific embodiment shown by the drawing.

What is claimed is: 1. A filter system for a cuit out said parasitic oscillations, said means comprising a connection point on the push-pull amplifler output circuit which is'at vsubstantially zero radio frequency potential with respect to ground, an inductance of 150 microhenries, a

resistance of 110 ohms, and a condenser oi 3'70 inicro-microfarads connected together in parallei and in series withthe connection point on said amplifier output circuit and ground.

2. In combination a pair of tubes each having an anode, a cathode and a grid, a tuned circuit connected across the grids, is tuned circuit consisting of inductance and capacity connected across the anodes, and means for preventing parasitics and permitting high efliciency operation at fundamental frequency, consisting of a circuit connected from the midpoint of said inductance across the anodes to ground, said circuit including a resistor, an inductance and a capacity connected in parallel.

3. A push-pull amplifier circuit filter system comprisinga pairof electron discharge devices each having an anode, a cathode and, a grid, a

tuned circuit connected across said grids, a tuned circuit consisting of an inductance and capacity connected across said anodes, a midpoint tap on said inductance, and means for preventing parasitics' and permitting high eiiicient operation at the fundamental frequency, said means including an inductance, a condenser and a resistance connected in parallel with each other and in se 'rieswith said mid-point tap and oneside of a condenser, the other side of said condenser being connected to ground.

' GEORGE L. USSELMAN. 

